1. Field of the Invention
The present invention relates to a PON system, a station side device and a redundant method used for the same, and specifically to a redundant method in a PON (passive optical network) system.
2. Description of the Related Art
As the Internet has become so popular in these days, it has been drawn attention to change over the conventional types of access line to broadband access lines. Various types of access line including the ADSL (asymmetric digital subscriber line), a cable modem and the like have been adopted for a broadband access line. Now, the PON (passive optical network) has drawn attention as an approach to widen the band. It is the worldwide trend to transfer to the PON. These days, the PON system with giga-bits of the Interface speed has been come into use.
The basic configuration of the PON system is shown in FIG. 37 and FIG. 38. In FIG. 37 and FIG. 38, ONUs (optical network unit: house device) 61 to 63 are set in end user's houses, and an OLT (optical line terminator: station device) 4 is set in a station. The ONUs 61 to 63 and the OLT 4 are connected via an optical fiber and an optical splitter: optical branching/coupling device) 5.
A personal computer (hereinafter referred to as a PC) at each end user's house is connected via each of the ONUs 61 to 63 to the network, and further connected to the upper level networks and the Internet via the OLT4. The wavelength of uplink signals (generally, with the wavelength of 1.3 μm) and the wavelength of downlink signals (generally, with the wavelength of 1.5 μm) are multiplexed. The OLT4 and the ONUs 61 to 63 are connected via an optical fiber.
The downlink signals are broadcasted from the OLT 4 to all the ONUs 61 to 63. Each of the ONUs 61 to 63 checks the destination of each frame and takes the frame destined to the self unit (see FIG. 37).
The uplink signals from each ONUs 61 to 63 converge at an optical splitter 5. Here, time division multiplexing is adopted to avoid signal collisions. Thus, the OLT 4 arbitrates an output request (REPORT) momently reported from each of the ONUs 61 to 63, calculates a transmitting time based on a distance between the OLT 4 and each of the ONUs 61 to 63, and then grants a permission to sending out the signals (GATE) to each of the ONUs 61 to 63 (see FIG. 38).
The output request (REPORT) includes a state of a queue in the buffer (the length of the queue) as information. The permission to sending out the signals (GATE) includes a time to start sending out the signals and a duration of sending out the signals for each priority of the signal. The ONUs 61 to 63 send out the uplink signals according to the GATE. That is, the uplink band allocation is realized by timeslot allocation.
FIG. 37 and FIG. 38 show a flow of signals where the OLT 4 accommodates one PON, with three ONUs 61 to 63 being connected to it. FIG. 37 shows the downlink signals and FIG. 38 shows the uplink signals. In FIG. 37 and FIG. 38, each number in a box indicates a frame of signals destined to each of the ONUs 61 to 63 and originated from each of the ONUs 61 to 63. Distances between the OLT 4 and respective ONUs 61 to 63 differ. The OLT 4 needs to measure a time for the signals to shuttle between itself and each of the ONUs 61 to 63 in advance when it starts up in order to grand an appropriate permission to sending out signals to each of the ONUs 61 to 63. That measurement is called the PON startup process.
The OLT 4, which is a station side device of the PON, generally has a plurality of PONs with functions of concentrating and distributing signals. That is, as shown in FIG. 39, the OLT 7 has a plurality of PON cards (PON interface section) #1 to #N+1 and a distributor/concentrator 71.
Each of the PON cards #1 to #N+1 has a function of inputting/outputting the PON, a function of terminating a signal, a function of interfacing internal device and the like. The distributor/concentrator 71 has a function of concentrating/distributing signals as well as a function of inputting/outputting interface with the next stage device. The OLT 7 shown in FIG. 39 has N+1 PON cards #1 to N+1, with each of the PONs being connected with three ONUs 9-1 to 9-3 N+3.
For the purpose of making a PON redundant to improve reliability of the system, an approach to duplex a PON is proposed (for example, see Japanese Patent Laid-Open No. 2000-349799 and Japanese Patent Laid-Open No. 2001-119345). An example of a configuration to duplex a PON is shown in FIG. 40 and FIG. 41. In the figures, two PON cards #0 and #1 are prepared for connecting the ONUs to a PON network via optical splitters 82 and 83.
FIG. 40 shows a configuration with the optical splitters 82 and 83 being set outside the OLT 80.
FIG. 41 shows a configuration with the optical splitter 92 being included in the OLT 90. In either case, the uplink signal on the PON network is branched into two to be input into two PON cards #0 and #1. The downlink signal is output from either of the two PON cards #0 and #1 to be sent onto the PON network.
The above-mentioned redundant method in a conventional PON system has a simple configuration to be realized easily, however, it needs to prepare two PON cards #0 and #1 at the side of the OLTs 80 and 90 for one PON network. That causes a drawback of increasing volumes of the OLTs 80 and 90 as well as their costs. With the volumes and costs being kept as they are, accommodating efficiency of each of the OLTs 80 and 90 degrades to half.
In the example shown in FIG. 39, half of the PON cards #1 to #N+1 is allocated to current use with the rest left as standby. In that example, the uplink signal is branched into two so that the power of light input into a PON card is reduced to half.
As a result, the example also has a restriction to further limit a distance between the OLT 7 and each of the ONUs 9-1 to 9-3 N+3 (i.e., the distance needs to be shorten), compared to a case where no redundant is taken.
Then, the object of the present invention is to provide a PON system, a station side device and a redundant method used for the same for solving the above-mentioned problems and realizing an economical and highly reliable system.